1. Field of the Invention
The present invention relates, in general, to a method for the preparation of tetragonal zirconia polycrystal composite and, more particularly, to a method for preparing a tetragonal zirconia polycrystal composite, capable of allowing the composite to resist low-temperature degradation yet to have high toughness.
2. Description of the Prior Art
Generally, zirconia exists in three allotropic forms: monoclinic, tetragonal and cubic and there is a large volume expansion during the phase transformation from tetragonal to monoclinic which occurs when tetragonal zirconia polycrystal (hereinafter referred to as "TZP") having high toughness and strength is exposed to a low temperature range of 100.degree. to 400.degree. C. for a long time in air. This phase transformation is accompanied by the generation of fine-scale cracks on the material surface. As a result, the strength becomes significantly decreased, which is a phenomenon called low-temperature degradation.
In order to secure reliability of a TZP material at low or high temperature, there is a need for a technology to effectively suppress or disrupt the low-temperature degradation. Recently, based on the need, several control processes for low-temperature degradation of TZP have been suggested.
Representative examples of existing control processes for low-temperature degradation of TZP include an increase of the amount of Y.sub.2 O.sub.3 content and a decrease in the average particle diameter of sintered bodies by lowering sintering temperature.
However, these conventional control processes for low-temperature degradation of TZP include such a problem that the resulted zirconia gets low toughness because they incapacitate the phase transformation to monoclinic, which leads to difficulty in stress-induced phase transformation, a toughening mechanism.
A representative for yttria (yttrium oxide, Y.sub.2 O.sub.3)-stabilized zirconia, commercially available, is 3Y-TZP, which comprises 3 mol % of yttria. The zirconia having such composition is known to not only show high toughness due to a high content, e.g. more than 90 %, of tetragonal phase but also exhibits superior strength because of a small average grain size, e.g. less than 0.5 .mu.m, of sintered bodies.
There is disclosed an improvement in toughness of TZP in U.S. Pat. No. 4,886,768 wherein a pentavalent oxide is used as a toughening agent. This patent says that the presence of 1.5 mol % or more of the pentavalent oxide such as Ta.sub.2 O.sub.5 or Nb.sub.2 O.sub.5 results in an increase in toughness of the order of a three-fold increase in fracture toughness relative to the absence thereof.
For all noticeable influence of low-temperature degradation on the reliability of substance in practical application of TZP, however, there is no mention thereabout in U.S. Pat. No. 4,886,768. The present inventors have tested the 3Y-TZP of this patent and found that it has a serious problem of low-temperature degradation.
Consequently, the addition of Nb.sub.2 O.sub.5 increases toughness according to U.S. Pat. No. 4,886,768 but yet effects degradation under the condition of low temperature treatment, thereby resulting in bad reliability of the material
U.S. Pat. No. 4,507,394 discloses that ceramics comprising a composition consisting essentially of ZrO.sub.2 and/or HfO.sub.2 added with 5 to 30 mol % of Y.sub.2 O.sub.3 and 5 to 40 mol % of Nb.sub.2 O.sub.5 or Ta.sub.2 O.sub.5 exhibit high electrical resistivity and mechanical strength. However, the toughness measurements of any of the ceramics having such composition is nowhere to be found in this patent. In addition, although it is claimed that the ceramics described by the patent have high strength, the highest strength measured for these materials is lower than those of other well-known TZP materials. Further, there is no description in this patent in conjunction with the preparation of a ceramic composite body high in toughness and resistant to low-temperature degradation by use of the composition.
U.S. Pat. No. 5,008,221 describes ceramic alloys comprising a mixture of a composition of Y.sub.2 O.sub.3 -stabilized TZP exhibiting high toughness through the inclusion of 0.5-8 mol % of YNbO.sub.4 or YTaO.sub.4, and ceramics containing such alloys as a toughening agent. In detail, the three-component system of ZrO.sub.2 --Y.sub.2 O.sub.3 --Nb.sub.2 O.sub.5 described is satisfied by a polygon O(4.85, 0.35), P(7.00, 2.50), Q(6.80, 2.50), R(7.80, 3.50), S(7.50, 3.50), T(8.00, 4.00), U(s4.40, 4.00), V(3.40, 3.00), W(3.50, 3.00), X(2.75, 2.25), Y(3.25, 2.25), Z(1.35, 0.35) of (mole % of Y.sub.2 O.sub.3, mole % of Nb.sub.2 O.sub.5) enclosing the composition region of the patent. Examples of ceramic matrices with which the ceramic alloys in the composition region of the polygon form the ceramic composite body include .alpha.-alumina, .beta.-alumina, Al.sub.2 O.sub.3 --Cr.sub.2 O.sub.3 solid solutions, mullite, sialon, nasicon, silicon carbide, silicon nitride, spinel, titanium carbide, titanium diboride, zircon, and zirconium carbide.
Meanwhile, a composition comprising 2 to 4 mol % of Y.sub.2 O.sub.3 and 0.5 to 3.0 mol % of Nb.sub.2 O.sub.5 or Ta.sub.2 O.sub.5, as claimed in the supra patent, is the same with that of previously mentioned U.S. Pat. No. 4,886,768. In addition, prevention of low-temperature degradation of the zirconia and the composite body claimed is mentioned nowhere in U.S. Pat. No. 5,008,221.
A test for low-temperature degradation was carried out for the ceramic compositions of U.S. Pat. No. 5,008,221 by the present inventors. In the test, significantly high toughness ceramic compositions selected from the ZrO.sub.2 --Y.sub.2 O.sub.3 --Nb.sub.2 O.sub.5 systems suggested by U.S. Pat. No. 5,008,221 were sintered at 1,550.degree. C. for 2 hours and then, subjected to heat treatment at 220.degree. C. for 120 hours in air. Results of the test are given as shown in the following Table 1. In this table, sample numbers are the same with those of the patent.
TABLE 1 ______________________________________ Results for Low-temperature degradation After Heat Treatment at 220.degree. C. for 120 hours in air. Sample mole % mole % mole % phase after % m-ZrO.sub.2 No. ZrO.sub.2 YO.sub.3/2 YNbO.sub.4 sintering after aging ______________________________________ 1 96.10 3.90 0.00 t 96 2 94.20 5.80 0.00 t 66 3 93.95 5.80 0.25 t + c 67 4 92.94 5.60 1.26 t + c 58 5 89.25 5.50 5.25 c + t 16 6 80.70 5.00 14.30 t + c 0 7 62.40 3.80 33.80 * .diamond-solid. 8 94.20 3.80 2.00 t 86 9 92.30 5.70 2.00 t + c 65 10 91.30 5.60 3.10 t + c 63 11 94.00 2.90 3.10 t + m .diamond-solid. 12 93.10 3.80 3.10 t 86 13 93.20 4.80 2.00 t + c 74 14 92.20 4.70 3.10 t + c 76 15 91.40 6.60 2.00 t + c 53 24 91.70 1.90 6.40 t 84 25 92.00 3.80 4.20 t + c .diamond-solid. 26 91.00 3.70 5.30 t + c .diamond-solid. 43 96.10 2.90 1.00 m + t 96 44 95.10 3.90 1.00 t 77 ______________________________________ note: m: mZrO.sub.2, t: tZrO.sub.2, c: cZrO.sub.2, *: nonsinterable, .diamond-solid.:cracking
From Table 1, it is apparent that all t-ZrO.sub.2 except the sample #6 comprising high toughness compositions claimed in the patent transforms into m-ZrO.sub.2 under low temperatures with low-temperature degradation occurring. It should be noted that the sample #6 does not consist of t-ZrO.sub.2 only but a phase mixture of t-ZrO.sub.2 and c-ZrO.sub.2.
In the meanwhile, the present inventors observed that sintering at 1,500.degree. C. for 1 hour allows 3Y-TZP samples comprising 1.5 mol % of Nb.sub.2 O.sub.5 to exhibit tetragonal phase but 3Y-TZP samples comprising more than 1.5 mol % of Nb.sub.2 O.sub.5 to display both tetragonal and monoclinic phases because of instability of tetragonal phase. From this observation, it is recognized that 1.5% by mole is the amount of Nb.sub.2 O.sub.5 which is optimal to raise the toughness.